1. Field of the Invention
This invention generally relates to a transfer ratio varying apparatus capable of varying a transfer ratio between a steering angle of a steering wheel and an angle of a wheel assembly, where the wheel assembly includes wheels and an axle connecting the wheels.
2. Description of the Related Art
Conventionally, a transfer ratio varying apparatus is described in Japanese patent Laid-Open Publication No. 2000-211541. The transfer ratio varying apparatus includes a steering shaft for transmitting a steering angle of a steering wheel, and an actuator connected to the steering shaft. The actuator includes a housing, an input shaft positioned for integral rotation with the housing and connected to the steering shaft, a motor fixed in the housing and including a shaft that is rotatable (hereafter “motor shaft”), an output shaft rotatably supported by the housing and transmitting an angle to a wheel assembly, where the wheel assembly includes wheels and an axle connecting the wheels, and a gear mechanism disposed between the motor shaft and the output shaft in the housing and increasing or decreasing a rotational angle of the motor shaft. The increased or decreased rotational angle of the rotary shaft is outputted to the output shaft.
Normally, the wheel assembly can receive the angle which was generated by a variable transfer ratio relative to the steering angle of the steering wheel. However, in times of emergency, such as a malfunction of an electronic control unit (ECU), the input shaft of the actuator and the motor shaft can lock up, thereby disabling variance of the transfer ratio.
In the meantime, another Japanese Patent Laid-Open Publication No. 2001-30921 describes a steering apparatus including a damper. This steering apparatus includes a flexible coupling, a first steering shaft connected to one side of the flexible coupling, a second steering shaft connected to an opposite side of the flexible coupling, and a stopper plate attached to the flexible coupling for restraining relative displacement in a torsion direction of the first and second steering shafts. The stopper plate is firmly fixed with a mass, which is a damper. The damper includes a solid metal body coated with a rubber outer layer.
According to the steering apparatus with the aforementioned structure, vibration, which may occur upon steering the steering wheel, can be absorbed by the damper, thereby leading to a reduction of in-vehicle noise. However, in the structure of Japanese Patent Laid-Open Publication No. 2001-30921, it is necessary to have two steering shafts, which is undesirable.
When the above-described transfer ratio varying apparatus was actually mounted on a vehicle, it became obvious that in-vehicle noise is caused due to vibration of the steering wheel. In order to overcome this drawback, it may be preferable that the transfer ratio varying apparatus possesses substantially similar structure as the above-described steering apparatus. That is, the steering shaft of the transfer ratio varying apparatus is divided between first and second steering shafts with a flexible coupling therebetween, where the damper can be firmly fixed with a stopper plate attached to the flexible coupling.
However, the inventors of the present invention experimentally verified that vibration of the steering wheel arises from the rotation of the motor shaft. More particularly, vibration along the motor shaft rotation becomes vibration of the actuator. The actuator vibration is transmitted to the steering wheel via the steering shaft. Therefore, even if the damper is disposed between the first and second steering shafts, the vibration caused due to the motor shaft rotation may be amplified via the first and second steering shafts, which are relatively long. As a result, still some concerns may remain that the in-vehicle noise is not sufficiently reduced.
In light of foregoing, a need thus exists for providing an improved transfer ratio varying apparatus capable of effectively reducing in-vehicle noise.